The present invention generally relates to coating deposition processes and equipment. More particularly, this invention relates to a method and masking assembly for selectively depositing a coating on a turbine airfoil while preventing deposition of the coating on a dovetail of the airfoil.
Components of gas turbine engines, such as the blades and vanes (nozzles) of the turbine section within a gas turbine engine, are often formed of an iron, nickel, or cobalt-base superalloy. A turbine blade has an airfoil against which hot combustion gases are directed during operation of the gas turbine engine, and whose surface is therefore subjected to severe attack by oxidation, corrosion and erosion. The blade further includes a root section separated from the airfoil by a platform. Turbine blades are commonly anchored to the perimeter of a rotor or wheel by forming the rotor to have slots with dovetail cross-sections, and forming the root section of each blade to have a complementary dovetail profile whose oppositely-disposed undulatory surfaces, generally characterized by alternating lobes and recesses, interlock with the dovetail slot of the rotor.
Due to the severity of their operating environments, turbine blades often require environmentally protective coatings on the surfaces of their airfoils and platforms exposed to the hot gas path. Diffusion coatings such as chromide, aluminide, and platinum aluminide coatings are widely used as environmental coatings in gas turbine engine applications because of their oxidation resistance. Such coatings, which are typically applied to the internal and external surfaces of a blade, are produced by a thermal/chemical reaction process that takes place in a reduced and/or inert atmosphere at a specified temperature. Common processes include pack cementation and noncontact vapor (gas phase deposition) techniques, and typically take place at processing temperatures of about a 1900° F. (about 1040° C.) or more. The dovetail of a turbine blade is typically machined prior to the diffusion coating process, and is not coated during coating of the airfoil so that the dovetail will properly assemble with the dovetail slot in the rotor during engine build.
Slurries, putties, and tapes have been widely used as masks to prevent coating deposition on the machined surfaces of blade dovetails. One approach is to cover the dovetail surfaces with a mask formed from a slurry paste, such as a mixture of nickel powders and an organic binder. The slurry paste may be applied with pneumatic injection equipment and then dried to form a solid mask. Alternatively, the blade dovetails can be dipped into the masking slurry, with multiple dips typically being required to form an effective mask with sufficient thickness. The masked dovetails are then often wrapped in a metal foil to contain the maskant during the coating process. With either approach, the solid mask must be mechanically removed after the coating process, such as by grit blasting, rotating wire brush, etc. To avoid the requirement of removing a solid maskant, the dovetail can simply be buried in a nickel powder without any binder, so that the powder forms a loose maskant that covers the dovetail during the coating operation. Still another alternative is to cast the slurry into thin film tapes that can be individually applied to the blade. While this approach is well suited for masking localized areas, tapes are not typically used as a primary method for masking the undulatory machined surfaces of a dovetail.
Significant shortcomings associated with the above-noted approaches include the preparation, application, and removal of the masking materials, which can be labor intensive and require the services of a skilled individual. As such, alternative masking techniques have been proposed. On such approach is taught in commonly-assigned U.S. Pat. Nos. 6,224,673, 6,579,567, and 6,821,564 to Das et al. These patents teach the use of a reusable fixture to enclose those portions of an article, such as a gas turbine blade, on which a coating is not desired. The fixture has an internal cavity and at least one aperture whose cross-section is substantially the same as a cross-section of the article to be coated. In the case of a blade, the aperture is sized to enable the entire dovetail of the blade to be inserted through the aperture into the internal cavity of the fixture, so that the platform seals against the exterior of the fixture. The fixture may include a holder to stabilize the dovetail within the internal cavity.
While the teachings of Das et al. overcome the shortcomings associated with the use of masking tapes, slurries, and other types of coatings, further improvements would still be desirable.